This invention relates to a device for tensioning yarn or the like and more particularly but not necessarily exclusively to a device for applying tension to individual yarns and the like during warping or direct yarn feed processes (and in other textile, or related activities), so that the applied tension can be automatically adjusted over a wide tension range, and to maintain a constant and uniform tension with the minimum of yarn damage, thereby improving woven or knitted fabric quality, reduce fault rates, and allow the machine to operate at higher speeds than otherwise achievable.
Wire is often braided by a weaving type of operation, or prepared, or fed into machines, and the term yarn as used herein is intended to also cover wire.
A xe2x80x9cwarperxe2x80x9d is a machine for preparing yarns ready for weaving by winding hundreds (sometimes thousands) of parallel yarns from individual bobbins onto a spool known as a beam or a warp beam. This beam forms the warp in weaving or warp knitting. In some instances, the yarns can be warped directly into the textile machine.
The bobbins are held in an ordered fashion on a large frame known as a creel and drawn through a tension unit (located as close as practicable to the bobbins) to add sufficient tension to allow the yarn to be kept under control. Creels can sometimes be 20 to 30 meters in length in order to accommodate large numbers of bobbins.
A long standing problem in the textile industry has been the control of yarn tension during these processes, and many types of tension units have accordingly been developed to maintain a constant and consistent tension in each yarn across the warp. In the past various mechanical arrangements have been introduced, and more recently, variable drag spools have been designed, based on using the yarn feed as the driving means for a small electric generator, usually a small motor. Yarn from the bobbin is trained around the periphery of the rotatable spool or capstan, and in response to the travel of this yarn, is made to rotate. The drag rate of the spool is varied by varying the electrical load on the generator, which in turn, increases/decreases the tension on the outgoing yarn.
Conventional rotors, however, make relatively heavy spools, being constructed from iron laminations to improve the operational efficiency.
Ideally, any spool in the path of the yarn should have a minimum mass, since high movements of inertia cause over tensions at start-up, overruns when stopping, and a poor frequency response to control signal changes.
According to one aspect of the present invention there is provided a device for applying tension to yarn or the like, comprising a stator and a rotor, wherein the rotor comprises an outer spool around which, in use, yarn or the like is passed and an inner part including a cylindrical sleeve-like portion and wherein the stator comprises outer pole piece defining means extending between the outer spool and the sleeve-like portion of the inner rotor part, inner pole piece defining means surrounded by the sleeve-like portion of the inner rotor part and means for producing a magnetic flux between the inner and outer pole piece defining means, the rotor being formed of or substantially formed of aluminium or an aluminium alloy.
Preferably, the magnetic flux producing means comprises one or more coils.
Such a device produces a torque resulting entirely from the generation of an eddy current circulating within the inner rotor part. In practice the yarn or the like is wound around the outer spool so that the magnetic flux applies a reaction torque to the yarn. If the coil means produces a rotating magnetic flux, the yarn or the like will be in tension even when the rotor is stationary. The lightweight aluminium or aluminium alloy rotor provides low inertia. The reaction torque applied to the yarn or the like can be varied by varying the current applied to the one or more coils.
Preferably, the inner rotor part is connected at one end to the outer spool and has at another end a radially inwardly extending flange which supports an outer race of a ball or roller bearing assembly. In this case, the inner rotor part may have at said one end a radially outwardly extending flange connected directly or indirectly to the outer spool.
Preferably, the one or more coils are mounted on a core which includes a plurality of radially outwardly extending coil mounting parts equal in number to a number of said coils. In this case, the outer pole piece defining means comprises a plurality of pole pieces equal in number to the number of coils and the outer pole pieces are connected to respective coil mounting parts of the core.
According to a second aspect of the invention there is provided a device for applying tension to yarn or the like, the device comprising a stator, a rotor and a transducer for sensing a tension in the yarn, wherein the rotor comprises an outer spool around which, in use, yarn or the like is passed and an inner part including a cylindrical sleeve-like portion, and wherein the stator comprises outer pole piece defining means extending between the outer spool and the sleeve-like portion of the inner rotor part, inner pole piece defining means surrounded by the sleeve-like portion of the inner rotor part and means for producing a magnetic flux between the inner and outer pole piece defining means, the rotor being formed of or substantially formed of aluminium or an aluminium alloy.
Preferably, the device further comprises comparator means for comparing the tension sensed by the transducer with a reference value and for varying a current applied to the magnetic flux producing means in response to a difference between the sensed tension and the reference value to maintain a tension in the yarn equal to said reference value.
The invention will now be more particularly described by way of example, with reference to the accompanying drawings.